Node overlay: factor out different functions of radius editing

This makes the code more readable and allows for easier modification.
Node overlay: confirm radius also with mouse click while editing

If the radius of a node is edited, by pressing 'o', a yellow circle surrounding
the radius is shown. Up to now a radius was only confirmed by pressing 'o'
again. Now also a mouse click will confirm the current radius.

This is related to issue acardona/CATMAID#854.